Fin and Blue whale acoustic presence and behaviour at a Mid-Atlantic migratory habitat, the Azores Archipelago

Fin (Balaenoptera physalus) and blue whales (B. musculus) produce diverse low- frequency calls. Through passive acoustic monitoring (PAM), the study of their acoustic presence and behaviour can offer valuable insights into the ecology of these highly mobile and elusive species. Building on previous studies in the Azores (Romagosa et al. 2020, 2021, 2024) this thesis applies similar methods to a new dataset collected using Ocean Bottom Seismometers (OBS) over a five-month period (August 2022 – January 2023). Song calls produced by males of both species were detected from autumn onwards, aligning with the patterns reported earlier. In fin whales, the different timing of the peak in 40 Hz food-associated call rates indicated inter-annual variability. Detailed analysis on fin whale song revealed high variability in song parameters. Notably, when compared to a two-decade dataset from the North Atlantic a rapid shift in inter-note intervals brough new information on how fin whale song may evolve and into vocal plasticity of this species. Additionally, a 9 Hz song component was recorded in blue whales for the first time in this region. This work supports the potential ecological role of the Azores as a mating habitat for fin and blue whales and expands current knowledge of their acoustic behaviour in a mid-Atlantic region.